The water constituting the secondary fluid of heat exchangers of nuclear power stations is vaporized in a steam generator, whose wall is penetrated by a water supply pipe. This pipe is terminated by a horizontal toroidal distributor extending externally along the cylindrical wall of the steam generator. The interior of the steam generator and the interior of the torus communicate with the aid of distribution tubes traversing the wall of the latter and located on its upper generatrix. Relatively frequently these tubes are crook-shaped, i.e., the end thereof outside the torus terminates in a circular arc which is radially oriented with respect to the torus. Depending on the particular case, the other end of the tubes penetrates the torus, possibly up the approximately the center of its section, or is simply flush with the inner surface of its wall. The latter construction is essential if the torus is completed by a tubular connecting section, which is fitted into the supply tube during assembly.
Such distributors assist uniform water flow in the steam generator, ensure that the supply water undergoes pressure drops limiting the turbulence of its flow and in particular, in the case of lowering the liquid level in the steam generator, prevent draining of the pipes which would result in water hammer.
Such an arrangement prevents this phenomenon, but does not prevent the possible appearance of thermal stratification under certain exceptional operating conditions of the steam generator.
Under transient or accidental operating conditions, it may be necessary to supply the steam generator with cold water at a very low flow rate. It is then found that this water, which has a higher density than the hot water which previously flowed in the distributor, does not mix with the latter and circulates in the bottom of the supply pipe and the torus. The hot water remaining at the top of the water supply pipe and the torus is replenished as a result of convection currents with the hot water outside the torus and consequently remains at high temperature. This hot water is only discharged in that part of the torus diametrically opposite to the supply pipe, where the two cold air streams join again, which increases their pressure. The cold water level locally rises opposite to the entrance into the torus until it reaches and fills the distribution tube located at this point and then the cold water passes into the steam generator. However, this phenomenon remains local and there is always an interface between the two superimposed water layers. This interface corresponds to a thermal level which can exceed 100.degree. C. in said pressurized water. The walls of the torus and the supply pipe are therefore exposed to very high stress concentrations at these points, which are made more dangerous by the fact that they work in fatigue. Fractures have been noted at the point where the supply pipe traverses the envelope of the steam generator and is welded thereto. The metal there is already subject to internal stresses and has less scope for deforming.